Magnetic tape media comprises a medium for storing large amounts of data, and typically comprises a plurality of data tracks that extend longitudinally along the tape. A tape head is employed for reading and/or writing data on the data tracks, and is typically shared between various data tracks or groups of data tracks, and is moved between tracks or groups of tracks in the lateral direction of the tape. The tape head typically comprises a number of separate elements which read and/or write data with respect to a number of parallel data tracks, and is provided with one or more separate servo read transducers, which are laterally offset from the read and/or write elements, so as to track follow a servo band and be guided along the data track or tracks. A servo band provides the servo guidance along any of several paths within the band, and the tape head is repositioned laterally within a servo band so that the read and/or write elements access different data tracks. The servo bands are continuous to provide constant track following. U.S. Pat. No. 5,121,270 attempted to employ a system called a “servo” for magnetic tape, but without servo bands, and without track following. Rather, the tape head was stepped between data tracks. Narrow (data track width) longitudinal binary patterns of blocks were provided, such that a “servo” transducer straddling two laterally adjacent binary patterns would indicate the track address from the binary patterns, and, if the amplitudes of each sensed binary block was one-half of full amplitude, the stepped head would theoretically be “on-track”, but the sensed servo signal would actually go on or off due to the absence of blocks at one or the other of the laterally adjacent binary patterns, making the patterns discontinuous. Further, since the blocks were of the same frequency, the transducer servo could not identify which side of the straddled line contained a sensed block, preventing track following. Only if the amplitudes were dramatically different, would the head be stepped a small amount and the patterns retested. Tape, even though it may be guided laterally, is subject to lateral shifting as it is moved longitudinally, such that establishing a stepped position of a head for each data track would require low densities of data tracks, and, in contrast, track following allows high densities of data tracks to be utilized. Servo bands, such as those of the incorporated '065 patent allow for track following for each of a number of paths across the servo band.
One type of servo system for magnetic tape media is one in which a plurality of separate servo bands are laterally positioned on the magnetic tape media. Each of the servo bands provides the servo guidance for a group of data tracks, and the servo transducer of the tape head is repositioned laterally within a servo band so the read and/or write elements access different data tracks, and is repositioned laterally to another servo band to access still further data tracks. In one example, the servo bands are spaced apart and the data tracks are located between the servo bands. To insure that the servoing is precise, two servo transducers may be provided at either end of the tape head, straddling the data read and/or write elements. The lateral positioning may be obtained from either or both servo bands. The servo bands are encoded with essentially identical patterns for determining lateral position, such that the bands are substantially indistinguishable.
The lateral positioning of the tape head is typically accomplished by actuators, which may have mechanical or electromechanical components. Once the proper lateral positioning of the tape head has been accomplished, as the servo information being sensed by the servo transducer indicates, minor adjustments of the head to follow lateral movement of the tape or of the tracks on the tape may be made. During track following, sticking or other failure of the mechanical or electromechanical components can be ascertained from failure of the sensed servo information to show any correction. Similarly, lateral repositioning of the tape head to different tracks within the same servo band is accomplished by a continuous adjustment of position within the servo band. Hence, any sticking or other failure of the mechanical or electromechanical components can be ascertained by failure of the sensed servo information to show the desired movement.
However, the lateral repositioning of the head between the servo bands is typically conducted by a coarse actuator which, also typically, operates in open loop without feedback. Thus, as the tape head is repositioned between the servo bands, there is no feedback from the servo information to indicate that the switch from one servo band to any other servo band was successful, and, if the servo bands are substantially indistinguishable, at the supposed completion of the lateral movement, the tape head may be positioned at the wrong servo band, and the servo information will not indicate an error.
One way of determining whether the lateral movement has caused the tape head to be positioned at the correct servo band, is to provide a separate “independent” sensor, for example, that determines the approximate lateral position of the head with respect to the tape. Such an independent sensor may comprise a coarse optical sensor that measures the physical position of the head. Such a coarse sensor cannot be used for track following, but provides a backup to the actual servo system. Such extra sensors add cost to a tape drive, which is always undesirable, if the extra cost can be avoided. Another example is shown by U.S. Pat. No. 6,169,640, in which timing based servo bands are longitudinally displaced or offset from each other, such that simultaneously sensing two adjacent servo bands allows the servo system to determine the longitudinal offset between servo bands, from which the data band location of the tape head can be determined. The system, however, requires that both servo bands be sensed simultaneously in order to establish a differentiation and make a determination.